Implement basics of #144 (watchdog)

One thing that I'm interested in is if this is required on k20-based boards different than Teensy. Without `watchdog::init(watchdog::Disabled)` the blink example will hang/missbehave if the values of `wait(...)` are bigger than let's say 1000. For values around `10` the fact that board is in reset loop is easy to overlook. Can anyone try this?

Other than this, as this is my first PR, please be gentle. :)
Seems like this actually unlocks the watchdog perohperal. Does it need to be public though?